Showing papers on "Glass transition published in 2011"

Silica-Like Malleable Materials from Permanent Organic Networks

Abstract: Permanently cross-linked materials have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized Non–cross-linked polymers and those with reversible cross-links are processable, but they are soluble We designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization and showed that they are insoluble and processable Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries

Glass transition and phase state of organic compounds: dependency on molecular properties and implications for secondary organic aerosols in the atmosphere.

Abstract: Recently, it has been proposed that organic aerosol particles in the atmosphere can exist in an amorphous semi-solid or solid (i.e. glassy) state. In this perspective, we analyse and discuss the formation and properties of amorphous semi-solids and glasses from organic liquids. Based on a systematic survey of a wide range of organic compounds, we present estimates for the glass forming properties of atmospheric secondary organic aerosol (SOA). In particular we investigate the dependence of the glass transition temperature Tg upon various molecular properties such as the compounds' melting temperature, their molar mass, and their atomic oxygen-to-carbon ratios (O : C ratios). Also the effects of mixing different compounds and the effects of hygroscopic water uptake depending on ambient relative humidity are investigated. In addition to the effects of temperature, we suggest that molar mass and water content are much more important than the O : C ratio for characterizing whether an organic aerosol particle is in a liquid, semi-solid, or glassy state. Moreover, we show how the viscosity in liquid, semi-solid and glassy states affect the diffusivity of those molecules constituting the organic matrix as well as that of guest molecules such as water or oxidants, and we discuss the implications for atmospheric multi-phase processes. Finally, we assess the current state of knowledge and the level of scientific understanding, and we propose avenues for future studies to resolve existing uncertainties.

Glassy dynamics and dynamical heterogeneity in colloids

Abstract: Concentrated colloidal suspensions are a well-tested model system which has a glass transition. Colloids are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high; the particles are roughly analogous to individual molecules in a traditional glass. Because the particle size can be large (100 nm - 1000 nm), these samples can be studied with a variety of optical techniques including microscopy and dynamic light scattering. Here we review the phenomena associated with the colloidal glass transition, and in particular discuss observations of spatial and temporally heterogeneous dynamics within colloidal samples near the glass transition. Although this Chapter focuses primarily on results from hard-sphere-like colloidal particles, we also discuss other colloidal systems with attractive or soft repulsive interactions.

Direct measurement of molecular motion in freestanding polystyrene thin films.

Abstract: An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with different dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at Tg,DSC – 25 K (Tg,DSC is the glass transition temperature of bulk polystyrene). At Tg,DSC – 5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are interpreted as indicating the presence of a high-mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at Tg,DSC.

Comparison between Dicationic and Monocationic Ionic Liquids: Liquid Density, Thermal Properties, Surface Tension, and Shear Viscosity

Abstract: We report the data of liquid density, thermal properties including glass transition temperature, melting point, and decomposition temperature, surface tension, and shear viscosity for imidazolium-based dicationic ionic liquids with the anions of bis(trifluoromethylsulfonyl)amide, bis(pentafluoroethylsulfonyl)amide, tetrafluoroborate, and nitrate. To find the unique and general features of the dicationic ionic liquids, data of their corresponding monocationic ionic liquids are also summarized. The results of the dicationic ionic liquids showed that the density was high; the glass transition temperature and melting point were high, and they were thermally stable. Also the surface tension was large, and the shear viscosity was high in comparison with the reference monocationic ionic liquids. The data of the physical properties including liquid density, surface tension, and shear viscosity of the ionic liquids were also compared with that of alkanediols and alkyl alcohols to find the alkyene-linker and alkyl-...

Coamorphous drug systems: enhanced physical stability and dissolution rate of indomethacin and naproxen.

Abstract: One of the challenges in drug development today is that many new drug candidates are poorly water-soluble, and one of the approaches to overcome this problem is to transfer a crystalline drug into its amorphous form, thus increasing dissolution rate and apparent solubility of the compound. In this study, a coamorphous drug/drug combination between the two nonsteroidal anti-inflammatory drugs, naproxen and γ-indomethacin, was prepared and investigated. At molar ratios of 2:1, 1:1 and 1:2, the drugs were quench cooled in order to obtain a coamorphous binary phase. Physical stability was examined at 277.15 and 298.15 K under dry conditions (phosphorus pentoxide) and analyzed with X-ray powder diffraction (XRPD). Intrinsic dissolution testing was carried out to identify dissolution advantages of the coamorphous form over its crystalline counterparts or amorphous indomethacin. Fourier transform infrared spectroscopy (FTIR) was used for analyses at the molecular level to detect potential molecular interactions. Differential scanning calorimetry (DSC) thermograms were employed to assess the glass transition temperatures (T(g)), and the results were compared with predicted T(g)s from the Gordon-Taylor equation. Results showed that naproxen could be made amorphous in combination with indomethacin while this was not possible with naproxen alone. Peak shifts in the FTIR spectra indicated molecular interactions between both drugs, and it is suggested that the two drugs formed a heterodimer. Therefore, samples at the 1:2 and 2:1 ratios showed recrystallization of the excess drug upon storage whereas the 1:1 ratio samples remained amorphous. Intrinsic dissolution testing showed increased dissolution rate of both drugs in the coamorphous form as well as a synchronized release for the 1:1 coamorphous blend. All T(g)s displayed negative deviations from the Gordon-Taylor equation with the 1:1 ratio showing the largest deviation. In a novel approach of predicting the glass transition temperature, the 1:1 drug ratio was inserted as an individual component in the Gordon-Taylor equation with the excess drug representing the second compound. This approach resulted in a good fit to the experimentally determined T(g)s.

Topological principles of borosilicate glass chemistry.

Abstract: Borosilicate glasses display a rich complexity of chemical behavior depending on the details of their composition and thermal history. Noted for their high chemical durability and thermal shock resistance, borosilicate glasses have found a variety of important uses from common household and laboratory glassware to high-tech applications such as liquid crystal displays. In this paper, we investigate the topological principles of borosilicate glass chemistry covering the extremes from pure borate to pure silicate end members. Based on NMR measurements, we present a two-state statistical mechanical model of boron speciation in which addition of network modifiers leads to a competition between the formation of nonbridging oxygen and the conversion of boron from trigonal to tetrahedral configuration. Using this model, we derive a detailed topological representation of alkali–alkaline earth–borosilicate glasses that enables the accurate prediction of properties such as glass transition temperature, liquid fragi...

Surface self-diffusion of an organic glass.

Abstract: Surface self-diffusion has been measured for an organic glass for the first time. The flattening of 1000 nm surface gratings of liquid indomethacin occurs by viscous flow at 12 K or more above the glass transition temperature and by surface diffusion at lower temperatures. Surface diffusion is at least 10(6) times faster than bulk diffusion, indicating a highly mobile surface. Our data suggest that surface diffusion is the leading mechanism of surface evolution for organic glasses at micrometer to nanometer length scales.

Sub-Tg Cross-Linking of a Polyimide Membrane for Enhanced CO2 Plasticization Resistance for Natural Gas Separation

Abstract: Decarboxylation-induced thermal cross-linking occurs at elevated temperatures (∼15 °C above glass transition temperature) for 6FDA–DAM:DABA polyimides, which can stabilize membranes against swelling and plasticization in aggressive feed streams. Despite this advantage, such a high temperature might result in collapse of substructure and transition layers in the asymmetric structure of a hollow fibers based on such a material. In this work, the thermal cross-linking of the 6FDA–DAM:DABA at temperatures much below the glass transition temperature (∼387 °C by DSC) was demonstrated. This sub-Tg cross-linking capability enables extension to asymmetric structures useful for large scale membranes. The resulting polymer membranes were characterized by swelling in known solvents for the un-cross-linked materials, TGA analysis, and permeation tests of aggressive gas feed stream at higher pressure. The annealing temperature and time clearly influence the degree of cross-linking of the membranes, and results in a sli...

Beating crystallization in glass-forming metals by millisecond heating and processing.

Abstract: The development of metal alloys that form glasses at modest cooling rates has stimulated broad scientific and technological interest. However, intervening crystallization of the liquid in even the most robust bulk metallic glass-formers is orders of magnitude faster than in many common polymers and silicate glass-forming liquids. Crystallization limits experimental studies of the undercooled liquid and hampers efforts to plastically process metallic glasses. We have developed a method to rapidly and uniformly heat a metallic glass at rates of 10^6 kelvin per second to temperatures spanning the undercooled liquid region. Liquid properties are subsequently measured on millisecond time scales at previously inaccessible temperatures under near-adiabatic conditions. Rapid thermoplastic forming of the undercooled liquid into complex net shapes is implemented under rheological conditions typically used in molding of plastics. By operating in the millisecond regime, we are able to “beat” the intervening crystallization and successfully process even marginal glass-forming alloys with very limited stability against crystallization that are not processable by conventional heating.

Mechanical Reinforcement in Polymer Melts Filled with Polymer Grafted Nanoparticles

Abstract: We critically leverage the fact that polymer grafted nanoparticles behave akin to block copolymers to systematically vary nanoparticle dispersion and critically examine its role on the mechanical reinforcement in liquid-polymer-based nanocomposites. The nanoparticles are noncontacting, and we perform rheological measurements 80K above the polymer’s glass transition temperature. Our rheology results unequivocally show that reinforcement is maximized by the formation of a transient, but long-lived, percolating polymer–particle network with the particles serving as the network junctions.

Glass Transition Temperature of Polymer Nanoparticles under Soft and Hard Confinement

Abstract: When confined to the nanoscale, the glass transition temperature (Tg) of polymer films can deviate substantially from the bulk, i.e., the Tg-confinement effect. Due to ease of processing most studies have focused on the thickness-dependent Tg of thin films, while few have focused on extending investigations beyond thin films to other geometries. As polymers confined to higher geometrical dimensionalities become the enabling material in technologies ranging from drug delivery to plastic electronics to ultrafiltration, a greater understanding of size effects on the Tg is warranted. Here, we investigate the effects of three-dimensional confinement on the Tg of polymer nanoparticles under soft and hard confinement and quantitatively compare our results to those of thin films to explore commonalities or differences between the Tg-confinement effect for polymers confined to different geometries. Via modulated differential scanning calorimetry, we show that Tg decreases with size for polystyrene (PS) nanoparticl...

Correlation between relaxations and plastic deformation, and elastic model of flow in metallic glasses and glass-forming liquids

Abstract: We study the similarity and correlations between relaxations and plastic deformation in metallic glasses (MGs) and MG-forming liquids. It is shown that the microscope plastic events, the initiation and formation of shear bands, and the mechanical yield in MGs where the atomic sites are topologically unstable induced by applied stress, can be treated as the glass to supercooled liquid state transition induced by external shear stress. On the other hand, the glass transition, the primary and secondary relaxations, plastic deformation and yield can be attributed to the free volume increase induced flow, and the flow can be modeled as the activated hopping between the inherent states in the potential energy landscape. We then propose an extended elastic model to describe the flow based on the energy landscape theory. That is, the flow activation energy density is linear proportional to the instantaneous elastic moduli, and the activation energy density ρE is determined to be a simple expression of ρ E = 10 11 G + 1 11 K . The model indicates that both shear and bulk moduli are critical parameters accounting for both the homogeneous and inhomogeneous flows in MGs and MG-forming liquids. The elastic model is experimentally certified. We show that the elastic perspectives offers a simple scenario for the flow in MGs and MG-forming liquids and are suggestive for understanding the glass transition,plastic deformation, and nature and characteristics of MGs

Combining renewable gum rosin and lignin: Towards hydrophobic polymer composites by controlled polymerization

Abstract: Rosin polymer-grafted lignin composites were pre- pared via ''grafting from'' atom transfer radical polymerization (ATRP) with the aid of 2-bromoisobutyryl ester-modified lignin as macroinitiators. Three different monomers derived from dehydroabietic acid (DA) were used for execution of grafting from ATRP, while DA was separately attached onto lignin by a simple esterification reaction. Kinetic studies indicated con- trolled and ''living'' characteristics of all monomer polymeriza- tions. Thermal studies indicated that rosin polymer-grafted lignin composites exhibited glass transition temperatures in a broad temperature range from � 20 to 100 � C. The grafting of both DA and rosin polymers significantly enhanced hydropho- bicity of lignin. Static contact angle measurement of water droplets showed � 90 � for all these rosin modified lignin com- posites. X-ray photoelectron spectroscopy demonstrated that the surface of rosin-lignin composites was dominated with chemical compositions originating from the hydrocarbon rich rosin moiety. The impartation of hydrophobicity of rosin into lignin provided excellent water resistance of this class of renewable polymers, as all rosin-modified lignin com- posites showed water uptake below 1.0 wt %. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 3728-3738, 2011

Characterization and properties of in situ emulsion polymerized poly(methyl methacrylate)/graphene nanocomposites

Abstract: Poly(methyl methacrylate) (PMMA)/graphene nanocomposites were prepared by in situ emulsion polymerization. Raman and Fourier transform infrared spectra showed that PMMA polymer contained partially reduced graphite oxide. Dynamic mechanical analysis and differential scanning calorimetry analysis showed that graphene in the PMMA matrix acted as reinforcing filler; it enhanced the storage moduli and glass transition temperatures of the nanocomposites. Thermogravimetric analysis showed that the thermal stability of the nanocomposites increased by ca . 35 °C. The electrical conductivity of nanocomposite with 3 wt.% graphite oxide was 1.5 S m −1 at room temperature.

Triptycene Polyimides: Soluble Polymers with High Thermal Stability and Low Refractive Indices

Abstract: A series of soluble, thermally stable aromatic polyimides were synthesized using commercially available five- and six-membered ring anhydrides and 2,6-diaminotriptycene derivatives. All of these triptycene polyimides (TPIs) were soluble in common organic solvents despite their completely aromatic structure due to the three-dimensional triptycene structure that prevents strong interchain interactions. Low solution viscosities (0.07−0.47 dL/g) and versatile solubilities allow for easy solution processing of these polymers. Nanoporosity in the solid state gives rise to high surface areas (up to 430 m2/g) and low refractive indices (1.19−1.79 at 633 nm), which suggest very low dielectric constants at optical frequencies. Polymer films were found to be amorphous. The decomposition temperature (Td) for all of the polymers is above 500 °C, and no glass transition temperatures can be found below 450 °C by differential scanning calorimetry (DSC), indicating excellent prospects for high-temperature applications. Th...